Siemens SITRANS P500, Operating Instructions Manual, Page: 171 / 238

Functional safety
7.2 Safety Integrity Level (SIL)
SITRANS P500 with HART
Operating Instructions, 12/2014, A5E02344528-09
171
Description
The following table shows the dependency of the SIL on the "average probability of
dangerous failures of a safety function of the entire safety-instrumented system" (PFD
AVG
).
The table deals with "Low demand mode", i.e. the safety function is required a maximum of
once per year on average.
Table 7- 1 Safety Integrity Level
SIL Interval
4 10
-5
≤ PFD
AVG
< 10
-4
3 10
-4
≤ PFD
AVG
< 10
-3
2 10
-3
≤ PFD
AVG
< 10
-2
1 10
-2
≤ PFD
AVG
< 10
-1
The "average probability of dangerous failures of the entire safety-instrumented system"
(PFD
AVG
) is normally split between the following three components:
Figure 7-2 PFD distribution
The following table shows the achievable Safety Integrity Level (SIL) for the entire safety-
instrumented system for type B devices depending on the safe failure fraction (SFF) and the
hardware fault tolerance (HFT).
● Type B devices include analog transmitters and shut-off valves with complex
components, e.g. microprocessors (also see IEC 61508, Section 2).
● For detailed information on values and hardware/firmware versions for your device, refer
to the manufacturer declaration for the device (Declaration of Conformity, Functional
Safety according to IEC 61508 and IEC 61511): Certificates
( http://www.siemens.com/processinstrumentation/certificates ).
SFF HFT for type B devices
0 1 (0)
1)
2 (1)
1)
< 60 % Not permitted SIL 1 SIL 2
60 to 90 % SIL 1 SIL 2 SIL 3
90 to 99 % SIL 2 SIL 3 SIL 4
> 99 % SIL 3 SIL 4 SIL 4
1)
Operational reliability in accordance with IEC 61511-1, Section 11.4.4